It often desirable to know the amplitude and phase separation between the two signal components that are received by any radio, which will be referred to as in phase (I) and quadrature (Q) components. These components should be exactly ninety degrees out of phase with each other, to maximize signal separation and minimize mirroring of one signal in the other. A receiver can be calibrated by knowing this phase separation to bring it close to this ideal.
One approach to determining the amplitude and phase separation is to feed back a version of a signal generated by the transmitter, when the radio has a transmitter. However the transmitter and/or the receiver may cause an observed amplitude and phase discrepancy. All such a test can initially tell is that there is a amplitude and phase discrepancy, not where it is coming from. With additional circuitry the test signal may be transmitted and looped back through the receiver and then circuit analytic approaches may be used to surmise where the amplitude and phase error is coming from and its magnitude at the source. While this can be made to work with additional circuitry and computation, it does so at a price of complexity for the calibration and by requiring a large number of samples to insure the needed accuracy, which in turn may lengthen the calibration time.
Worse yet, several kinds of radios lack a transmitter, such as a Global Positioning System (GPS) receiver, an Amplitude Modulated (AM) receiver, a mobile TV receiver, a Frequency Modulated (FM) receiver, and a Television receiver. They have no transmitters that can be used to calibrate their amplitude and phase separation. Inexpensive, accurate mechanisms and amplitude and phase calibration methods are needed for receivers without requiring a transmitter.